1. Field of the Invention
The present invention relates to a method for producing a powder molded product from a slurry containing a powdery molding raw material, and to a powder molded product produced through molding of the slurry.
2. Description of Related Art
There have been known various methods for producing such a molded product. Among these methods, widely employed is a technique in which a primary molded product (a preliminary molded product differing from the below-described “green product,” which is a final powder molded product just before sintering) is formed through a widely known technique (e.g., injection molding or sheet molding) from a slurry containing raw material powder, an organic binder, and a dispersion medium, and the primary molded product is dry-solidified to thereby produce an unsintered powder molded product (hereinafter referred to as a “green product” (see, for example, Japanese Patent Application Laid-Open (kokai) No. H08-238613). Thus, when such a green product is subsequently sintered, a molded product of interest is produced.
Such a conventionally well-known production method poses a problem in that difficulty is encountered in producing molded products of complex shapes and that when the green product is formed through dry-solidification, relatively large shrinkage occurs in the molded product (hereinafter such shrinkage may be referred to as “drying shrinkage”). Such drying shrinkage is mainly caused by a reduction in distance between organic binder molecules associated with volatilization of a dispersion medium. When relatively large shrinkage occurs in a powder molded product, naturally, a large “variation” occurs in the degree of shrinkage (hereinafter may be referred to as “percent drying shrinkage”) of the molded product. When the percent drying shrinkage of a green product is high, or the “variation” of the percent drying shrinkage is large, a final molded product produced through sintering of the green product exhibits poor dimensional accuracy.
In order to solve such a problem, the present applicant previously proposed a method for producing a powder molded product (green product) through so-called “gel cast molding” (see, for example, Japanese Patent Application Laid-Open (kokai) No. H11-48222, H11-302302, 2001-335371, or 2010-241129, or International Publication WO 2009/104703). The production method disclosed in International Publication WO 2009/104703 includes the following steps: (1) preparing a slurry containing raw material powder, an isocyanate, a polyol, a urethane reaction catalyst, a dispersion medium, and a dispersant; (2) molding the slurry into a product through any known technique; and (3) dry-solidifying the resultant molded product.
In such a production method, when the molded product is dry-solidified, a urethane resin serving as an organic binder is produced through chemical reaction (urethane reaction) between unreacted isocyanate and polyol, resulting in gelation (solidification) of the slurry.
Through gel cast molding, molded products of complex shapes can easily be produced, since a slurry which is not gelled and has high fluidity is cast-molded. Through gelation after cast molding, a green product having a sufficient strength for enduring rough handling after mold releasing. As this gelation proceeds, cross-linking occurs between urethane groups (—O—CO—NH—) of adjacent urethane resin molecules. This cross-linking forms a strong network between urethane resin molecules. Therefore, even when volatilization of the dispersion medium proceeds, the distance between urethane resin molecules is less likely to be reduced, and thus the molded product exhibits low percent drying shrinkage.